[clang] [llvm] Enable ClangIR Build By Default- (PR #221291)

[Ambrose Leeb via cfe-commits]

================
@@ -842,6 +837,11 @@ The `alpha.cplusplus.UseAfterLifetimeEnd` checker was renamed to `alpha.core.Use
 
 #### Improvements
 
+### Clang IR Improvements
----------------
Sirraide wrote:

If you’re adding a new section, you might want to update `docs/ReleaseNotesTemplate.txt` as well.

https://github.com/llvm/llvm-project/pull/221291